An electrophotographic light-sensitive material may have various structures depending upon the characteristics required or an electrophotographic process to be employed.
An electrophotographic system in which the light-sensitive material comprises a support having thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface thereof is widely employed. The electrophotographic light-sensitive material comprising a support and at least one photoconductive layer formed thereon is used for the image formation by an ordinary electrophotographic process including electrostatic charging, imagewise exposure, development, and, if desired, transfer.
Furthermore, a process using an electrophotographic light-sensitive material as an offset master plate precursor for direct plate making is widely practiced.
Binders which are used for forming the photoconductive layer of an electrophotographic light-sensitive material are required to be excellent in the film-forming properties by themselves and the capability of dispersing a photoconductive powder therein. Also, the photoconductive layer formed using the binder is required to have satisfactory adhesion to a base material or support. Further, the photoconductive layer formed by using the binder is required to have various excellent electrostatic characteristics such as high charging capacity, small dark decay, large light decay, and small fatigue due to previous light-exposure and also have excellent image forming properties, and the photoconductive layer stably maintains these electrostatic characteristics against a change of humidity at the time of image formation.
Binder resins which have been conventionally used include silicone resins (e.g., JP-B-34-6670, the term "JP-B" as used herein means an "examined Japanese patent publication"), styrene-butadiene resins (e.g., JP-B-35-1960), alkyd resins, maleic acid resins, polyamides (e.g., JP-B-35-11219), polyvinyl acetate resins (e.g., JP-B-41-2425), vinyl acetate copolymers (e.g., JP-B-41-2426), acrylic resins (JP-B-35-11216), acrylic acid ester copolymers (e.g., JP-B-35-11219, JP-B-36-8510, and JP-B-41-13946), etc.
However, in the electrophotographic light-sensitive materials using these binder resins, there are various problems such that 1) the affinity of the binder with photoconductive powders is poor thereby reducing the dispersibility of the coating composition containing them, 2) the charging property of the photoconductive layer containing the binder is low, 3) the quality (in particular, the dot image reproducibility or resolving power) of the image portions of duplicate images is poor, 4) the image quality is liable to be influenced from the environmental conditions (e.g., high temperature and high humidity or low temperature and low humidity) at the formation of duplicate images, and 5) the photoconductive layer is insufficient in film strength and adhesion, which causes, when the light-sensitive material is used for an offset master, peeling off of the photoconductive layer, at offset printing resulting in decrease of the number of prints.
For improving the electrostatic characteristics of a photoconductive layer, various approaches have hitherto been taken. For example, incorporation of a compound having an aromatic ring or a furan ring containing a carboxy group or a nitro group either alone or in combination with a dicarboxylic anhydride in a photoconductive layer is disclosed in JP-B-42-6878 and JP-B-45-3073. However, the thus improved electrophotographic light-sensitive materials are yet insufficient in electrostatic characteristics and, in particular light-sensitive materials having excellent light decay characteristics have not yet been obtained. Thus, for compensating the insufficient sensitivity of these light-sensitive materials, an attempt to incorporate a large amount of a sensitizing dye in the photoconductive layer has been made. However, light-sensitive materials containing a large amount of a sensitizing dye undergo considerable deterioration in whiteness, which means reduced quality as a recording medium, and sometimes deterioration in dark decay characteristics occurs, thereby satisfactory reproduced images are not obtained.
On the other hand, JP-A-60-10254 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method of controlling the average molecular weight of a resin to be used as a binder resin for a photoconductive layer. More specifically, JP-A-60-10254 discloses a technique for improving the electrostatic characteristics (in particular, reproducibility as a PPC light-sensitive material on repeated use), and moisture resistance of the photoconductive layer by the combination use of an acrylic resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4 and an acrylic resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.4 to 2.times.10.sup.5.
Furthermore, lithographic printing master plates using electrophotographic light-sensitive materials have been extensively investigated. As binder resins for a photoconductive layer having both the electrostatic characteristics as an electrophotographic light-sensitive material and the printing characteristics as a printing master plate, there are, for example, a combination of a resin having a molecular weight of from 1.8.times.10.sup.4 to 10.times.10.sup.4 and a glass transition point (Tg) of from 10.degree. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid and a copolymer composed of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in JP-B-50-31011, a terpolymer containing a (meth)acrylic acid ester unit with a substituent having a carboxylic acid group at least 7 atoms distant from the ester linkage as disclosed in JP-A-53-54027, a tetra- or pentapolymer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in JP-A-54-20735 and JP-A-57-202544, and a terpolymer containing a (meth)acrylic ester unit with an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxyl group as disclosed in JP-A-58-68046. These resins are described to be effective to improve an oil desensitizing property of the photoconductive layer.
However, none of these resins proposed have proved to be satisfactory for practical use in electrostatic characteristics such as charging property, dark charge retention, and photosensitivity, and the surface smoothness of the photoconductive layer.
Also, the practical evaluations on conventional binder resins which are said to be developed for electrophotographic lithographic master plates have found that they have problems in the aforesaid electrostatic characteristics, background staining of prints, etc.
For solving these problems, JP-A-63-217354 and JP-A-64-70761 disclose that the smoothness and the electrostatic characteristics of a photoconductive layer can be improved and images having no background staining are obtained by using a low-molecular weight resin (molecular weight of from 1,000 to 10,000) containing from 0.05 to 10% by weight a copolymer component having an acidic group at the side chain of the copolymer and by using the same resin but having an acidic group at the terminal of the main chain of the polymer as the binder resin, respectively, and also U.S. Pat. No. 4,871,638, JP-A-63-220148, JP-A-63-220149, JP-A-1-100554, JP-A-1-102573, and JP-A-1-116643 disclose that the film strength of a photoconductive layer can be sufficiently increased to improve the printing durability without reducing the aforesaid characteristics by using the aforesaid low-molecular weight resin in combination with a high-molecular weight resin (molecular weight of 10,000 or more) and by utilizing a cross-linking reaction, respectively.
However, it has been found that, even in the case of using these resins, it is yet insufficient to keep the stable performance in the case of greatly changing the environmental conditions from high-temperature and high-humidity to low-temperature and low-humidity. In particular, in a scanning exposure system using a semiconductor laser beam, the exposure time becomes longer and also there is a restriction on the exposure intensity as compared to a conventional overall simultaneous exposure system using a visible light, and hence a higher performance has been required for the electrostatic characteristics, in particular, the dark charge retention characteristics and photosensitivity.
Further, when the scanning exposure system using a semiconductor laser beam is applied to hitherto known light-sensitive materials for electrophotographic lithographic printing master plates, various problems may occur in that the difference between E.sub.1/2 and E.sub.1/10 is particularly large and thus it is difficult to reduce the remaining potential after exposure, which results in severe fog formation in duplicated images, and when employed as offset masters, edge marks of originals pasted up appear on the prints, in addition to the insufficient electrostatic characteristics described above.